64 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



tous] threads passing to the peculiar body, and the vitality of the colony will, both 

 in times of full and diminished vigor, be increased by the avicularia; for the con- 

 stant snapping of the mandibles often continues" when the polypides are not extend- 

 ing themselves out the zooecia, and, as before said, even when there are few or 

 no polypides. Sections often show the avicularia in unchanged conditions, when 

 the zooecia only contain hystolysed remains of polypides." (Waters, 1904.) 



The avicularia are not protective organs, as former authors believed; in fact, 

 they are developed and much elongated in the most protected part of the zoarium. 

 This phenomenon is of constant occurrence in the Cellepores. 



The avicularia develop very often in the place of the tremopores and areolae; 

 the zoologists have figured many examples of them ; we ourselves have been able to 

 observe such occurrences on Schizopodrella linea Lonsdale, 1845, and on Enoplo- 

 stomella synthetica Canti and Bassler, 1917. 



There are three principal kinds of avicularia articulated, frontal or immersed, 

 and interzooecial. 



The articulated avicularia exist on the articulated zoaria ; they are. often very 

 complicated and quite perfected organisms (fig. 13. A). 



Frontal or immersed avicularia. These are quite variable; it is always possible 

 to distinguish the corneo-chitinous mandible, the calcified beak, the membranous 

 frontal area, and a mandibular cavity. (Fig. 13, B.) These are quite small and 

 simple, without pivot or denticle, hardly distinct from the areolae. 1 Others, on the 

 contrary, are highly perfected. (Fig. 13, C.) The glands mentioned by Waters 

 have unknown functions. 



Interzooecial avicularia. There are ordinary zooecia deprived of polypide and 

 containing only muscles. (Fig. 13, E.) Their variations are very important, for 

 they express corresponding anatomical peculiarities. Their frontal is chitinous in 

 the Malacostega (fig. 13, /*"), calcified in the Coilostega; in the latter case they 

 constitute the onychocellaria (figs. 13, G, H) of Jullien, and in the group of the 

 Tubifera they form the reticuloceUaria of Canu (fig. 13 /). They are straight and 

 symmetrical if the polypide of the adjacent zooecium has its large retractor muscle 

 placed at the middle of the base (fig. 13, //) : they are unsymmetrical if the same 

 muscle is attached laterally, as in the genus Onychocella (fig. 13, G). In the Asco- 

 phora the interzooecial avicularia occur chiefly in the family Adeonidae. Never- 

 theless it is not rare to find some zooecia provided with a mandible (Porella planu- 

 lata, new species, MetroperieUa grandipora, new species, etc.). (Fig. 13, J.) 



Vibracula. The vibracula are heterozooecia formed of a cavity with chitinous 

 or calcareous walls and of a long cilium or seta. Their organization is identical 

 with that of the avicularia; they differ only in the articulation of the seta (=whip 

 or flagellum) and in the great length of the latter. 



" The base of the asymmetrical seta of the vibracula is very complicated with 

 a large number of curiously shaped protuberances, to some of which the muscles 



1 On the fossils it is often impossible to say if an observed small pore is a tremopore, an avicularium, a 

 vibraculum, or a radicular pore. 



